1. Field of the Invention
The present invention relates to a heating device for a hygrometric sensor for automatically controlling vehicle equipment, in particular windshield wipers.
2. Description of the Prior Art
French Utility Certificate No. FR-A-2 498 137 and French Pat. No. FR-A-2 555 752, in particular, disclose automatic controls of vehicle equipment, for example the windshield wipers. Such controls, set into action by the atmospheric elements to which the vehicles are exposed, must ensure operation of the windshield wipers, lighting of the low-beam or fog lights or, more generally, must ensure operation of the equipment for facilitating driving in bad weather.
The automatic controls of this type are known to be controlled by a hygrometric sensor constituted essentially by an electric resistor variable as a function of the degree of humidity due to the inclement weather to which the vehicle is exposed, for example fog or rain.
This hygrometric sensor, generally constituted by a printed circuit on a support made of plastic material, is mounted at the front of the vehicle in a region exposed directly to the ambient humidity, but precautions are taken to avoid it being reached by a stream of water flowing over the bodywork of the vehicle.
Furthermore, it is known that, for the automatic control to function suitably, it is necessary to ensure a permanent heating of the support of the hygrometric sensor. To that end, at least one electrical heating resistor is embedded in the support of the sensor.
In the case of the two known embodiments mentioned hereinabove, this heating resistor is generally switched on by means of an electrical voltage which varies as a function of the degree of humidity detected by the sensor. Prohibitive overheating of the support of the sensor, which would risk deteriorating it, is thus avoided.
However, the heating device which has just been described presents a drawback. If a vehicle has remained parked in a damp place, for example a garage, the sensor causes operation of the equipment when the driver starts up his vehicle, even if there is no humidity deposited on the windshield which would hinder driving. The various equipment controlled thereby, particularly the windshield wipers, continue to operate, sometimes for several minutes, until the support of the sensor is sufficiently heated and all dampness on the detector resistor has disappeared.
This drawback is due to the fact that the power of heating is lower as the rate of humidity detected is itself low, as is the case in a garage.
Utility Certificate No. FR-A-2 498 137 has certainly proposed using heating means independent of the degree of humidity and without any temperature monitoring. However, such a solution is unsatisfactory, as the heating resistor must still be such that there is no risk of deterioration of the support of the sensor, i.e. the heating will not be intense. The drawback set forth hereinabove, when a vehicle is started up in a sheltered but damp place, leads to unnecessary and relatively long operation of the equipment, and in particular of the windshield wipers.